1. Field of Invention
The present invention relates to controlling the power factor of an alternating current/direct current, silicon controlled rectifier drilling rig such as are used for drilling oil and gas wells.
2. Description of Prior Art
In the petroleum industry, the apparatus and machinery used to drill wells is commonly known as a drilling rig or a rig. Over the years, various types of rigs have been used by the industry and have been classified either by reference to the type of power used on board the rig to provide the motive force necessary to turn the drill bit or perform the other rig operations or as to the type of terrain on which the rig is situated. For example, a rig may be termed an "offshore" rig if it is one used for offshore drilling, but more commonly rigs are referred to as mechanical, DC/DC "Ward-Leonard" or AC/DC rigs depending upon the type of power coupling used to provide motive force for the drilling operations. Mechanical rigs used dedicated diesel engines which were clutched to mechanically engage various aspects of the rigs machinery to perform the operations of drilling, pumping, and lifting the drill string when appropriate. DC/DC rigs used dedicated diesel-electric motor/generator combinations to perform each of the necessary drilling functions in the same fashion as the mechanical-clutched rigs.
For the last decade, many AC/DC rigs were built using silicon controlled rectifier AC to DC converters, and hence have been referred to as "SCR" rigs. AC/DC or "SCR" rigs use a pool of diesel engine-AC generator sets to provide alternating current power to a rig bus from which DC motors or other AC or DC equipment draw power to perform various rig functions. Recently, because of their inherent advantages, SCR rigs have become preferred by the industry. SCR rigs are more tolerant to the loss of a particular diesel engine due to a maintenance requirement or similar problem, are less prone to maintenance difficulties in general, are easily assembled on a drilling rig and are generally less expensive to set up and operate than other types of rigs.
SCR rigs typically used three phase, full control, line commutated silicon controlled recifier, two-way AC to DC converter bridges. These SCR converter bridges operated at a power factor which was approximately equal to the ratio of actual output to full voltage output capability. Because these converters were line commutated, the fundamental component of current always lagged the voltage, hence a lagging power factor was characteristic of SCR rigs. SCR rigs used electric DC motors to drive various rig tools or functions such as mud pumps which were used to provide drilling fluid during drilling operations. Because electrically driven mud pumps required high pressure (high current), low speed (low voltage) operation, it was not uncommon that rigs operated during pumping operations at power factors of 0.4 or 0.5 lagging. The operation at such low power factors frequently required that more diesel engine-generator sets than necessary to provide the real power requirements be operated solely to provide the necessary reactive power (Kilovolt-amperes-reactive, hereinafter KVAR) because of the poor power factor of the operating rig. Thus, SCR drilling rigs typically operated at very inefficient levels consuming excess fuel simply to have standby KVAR available when required. The problem was aggrevated due to the specific fuel consumption characteristics of diesel engines which typically consume some fuel per horsepower-hour when lightly loaded than when fully loaded.
Another problem encountered during drilling operations is related to the need to be able to lift up on the drill string without shutting down mud pumps to make a connection or lengthen the drill string. In the past, due to an insufficient KVAR supply the lifting capacity of the rig was limited during pumping operations. In order to add more string to the drill string, it was necessary to either shut down the pumps or bring an additional engine-generator set on line to provide for peak power demands during operation of the draw works and pumps simultaneously, or during tripping of the drill string.
Generally speaking, the prior art has attempted to solve the problem presented during peak demand operations due to poor power factors in one of three ways:
1. The two motors driving mud pumps were connected in series to limit the current demand placed upon the power generation system. This solution was obviously not effective on single motor mud pumps, or when as commonly occurred, pumps had to be run at a greater than 50% speed to produce the required volume. Furthermore, even if pumps were placed in series, it was still necessary to provide additional engine-generator sets to provide KVAR for the draw works during tripping operations or when making additional connections.
2. Banks of capacitors were installed on the rig bus to supply a fixed amount of leading KVAR. This attempted solution also had several disadvantages. At low loads, the corrected power factor could be as poor leading as a result of the added KVAR as it was lagging without the compensation by the capacitors. Because the available power factor compensation was voltage dependent, and an increased KVAR demand (low voltage) was not met by an increased capability to compensate the power factor, voltage regulation was adversely affected. Furthermore, system short circuit current was significantly increased, often beyond the original rig design limits, and the introduction of capacitance gave the system both sub-synchronous and super-synchronous resonant frequencies not easily calculated but within the range of excitation by the SCR drive system, thereby creating potential system stability problems.
3. The rig generators were oversized, such that it was not uncommon to find 1500 KVA generators on 850 KW engines. Even this solution was not often sufficient and was expensive when done for all engine-generator sets. Aside from the higher initial capital expense required to provide oversize generators, the operation of oversized lightly loaded generators was inherently inefficient.